Three-level converters are converters that have three DC poles. In addition to positive and negative DC poles, they have a neutral DC pole. Examples of three-level neutral-point-clamped converters including an example of an active three-level neutral-point-clamped converter are given in T. Brückner, S. Bernet and H. Güldner, “The Active NPC Converter and Its Loss-Balancing Control”, IEEE transactions on industrial electronics, Vol. 52, No. 3, June 2005.
FIG. 1 shows an example of a switching branch of a three-level converter in accordance with a known implementation. The switching branch includes six diodes D1 to D6 and six controllable switches S1 to S6. A converter including one or more switching branches, like that of FIG. 1, can operate as a rectifier or as an inverter. The switches S1 to S6 are then controlled according to a modulation scheme used.
In a rectifier operation of the switching branch, when a current flows into the switching branch through an AC pole AC, the AC pole AC can be connected to a positive DC pole Udc+ via the first diode D1 and the second diode D2 and to a neutral DC pole NP either via the second diode D2 and the fifth switch S5 or via the third switch S3 and the sixth diode D6. Further in a rectifier operation of the switching branch, when a current flows out of the switching branch through the AC pole AC, a negative DC pole Udc− can be connected to the AC pole AC via the fourth diode D4 and the third diode D3 and the neutral DC pole NP can be connected to the AC pole AC either via the sixth switch S6 and the third diode D3 or via the fifth diode D5 and the second switch S2.
In an inverter operation of the switching branch, when a current flows into the switching branch through the AC pole AC, the AC pole AC can be connected to the negative DC pole Udc− via the third switch S3 and the fourth switch S4 and to the neutral DC pole NP either via the third switch S3 and the sixth diode D6 or via the second diode D2 and the fifth switch S5. Further in an inverter operation of the switching branch, when a current flows out of the switching branch through the AC pole AC, the positive DC pole Udc+ can be connected to the AC pole AC via the first switch S1 and the second switch S2 and the neutral DC pole NP can be connected to the AC pole AC either via the fifth diode D5 and the second switch S2 or via the sixth switch S6 and the third diode D3.
The switching branch of FIG. 1 can be implemented as one integrated power module that includes the diodes and controllable switches of the main circuit. Cooling optimization may then present a problem, e.g. because different components can have different specifications for cooling.
One solution to this problem is to distribute the loading to different components in order to distribute the power dissipation between the components. Thus, the connection between the neutral DC pole NP and the AC pole AC can be alternated between the alternative connection routes as described above.